Add balloon_min to VM.getVmBalloonInfo

The balloon_min key will take its value from memGuaranteedSize
attribute. It's default value will be 0, but can be overriden by
whoever is creating the VM instance or setting the attributes.

This functionality is needed for basic memory QoS.

Change-Id: I4fca97bf0d3621883d1320673c648d0fbbd50340
Signed-off-by: Martin Sivak <msivak@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/15799
Reviewed-by: Dan Kenigsberg <danken@redhat.com>